The fundamental problem in optical metrology is interferometric phase estimation. Distance measuring interferometers, ellipsometers, flying height testers and other optical instruments depend on an accurate measurement of relative phase between two orthogonal components of a polarized test beam of light. A device or subsystem that performs this task will be referred to herein as an interferometric receiver. Modern interferometric receivers must perform at high speed, with excellent repeatability and linearity. A further problem in optical metrology is the measurement of the intensities of two orthogonally polarized components of a polarized test beam. An example of a technology for which the interferometric receiver must provide both phase and intensity information for the two polarization components is taught in my commonly owned, co-pending U.S. patent application entitled "Optical Gap Measuring Apparatus and Method" filed Mar. 22, 1995 and bearing U.S. Ser. No. 08/408,907.
There are frequent references in the art to heterodyne methods of phase estimation, in which the phase varies with time in a controlled way. For example, in a known form of prior-art heterodyne distance-measuring interferometer, the source emits two orthogonal polarizations having slightly different optical frequencies (e.g. 2 MHz). The interferometric receiver in this case is typically comprised of a linear polarizer and a photodetector to measure the time-varying interference signal. The signal oscillates at the beat frequency, and the phase of the signal corresponds to the relative phase difference. A further representative example of the prior art in heterodyne distance-measuring interferometry is taught in commonly-owned U.S. Pat. No. 4,688,940 to G. E. Sommargren and M. Schaham (1987). On the one hand, an important advantage of the heterodyne technique is that the interferometric receiver is simple to construct and to calibrate. On the other hand, the heterodyning requires a specialized source, such as a Zeeman-split HeNe laser, or a high-speed modulator. Further, heterodyne interferometric receivers do not provide any information regarding the intensities of two orthogonally polarized components of a polarized test beam.
There are also frequent references in the art to homodyne methods of phase estimation, which uses a single-frequency source together with an interferometric receiver comprised of a plurality of photodetectors corresponding to static phase shifts. In a typical homodyne receiver, the phase estimation is performed by introducing static phase shifts via polarizing components such as wave plates. Representative prior art includes regarding such homodyne methods U.S. Pat. No. 5,374,991 to L. G. Atkinson, K. J. Vent, J. P. Wong, U.S. Pat. No. 5,018,862 to M. F. Aiello, and U.S. Pat. No. 5,392,116 to G. Makosch. Additional prior art regarding homodyne techniques is taught in an article entitled "A low cost laser interferometer system for machine tool applications" by A. Dorsey, R. J. Hocken and M. Horowitz (Precision Eng. 5 p.29, 1983), in an article entitled "Instantaneous phase measuring interferometry" by R. Smythe and R. Moore (Proc. Soc. Phot. Opt. Instr. Eng. 429 p.16, 1983) and in an article entitled "Accurate polarization interferometer" by V. Greco, G. Molesini, F. Quercioli (Rev. Sci. Instrum. 66 p.3729, 1994). The advantage of the homodyne technique is that it does not require a frequency difference between the polarization components of the test beam. However, there is a much greater concern with respect to the fidelity of the polarizing components and the differences in photodetector response. Generally, prior-art homodyne receivers are inaccurate, difficult to align and require expensive components. Prior-art homodyne receivers also do not provide any information regarding the relative strengths of the two orthogonal polarization components of the test beam.
The prior art provides some examples of calibration techniques which have been employed in an effort to improve homodyne receiver performance. A representative technique is taught in an article entitled "Determination and correction of quadrature fringe measurement errors in interferometers" by P. L. M. Heydemann (Appl. Opt. 20, p.3382, 1981). However, the prior art method taught by Heydemann applies to a simple quadrature homodyne receiver having only two detectors. This two-detector receiver is known to be disadvantageously sensitive to intensity fluctuations in the test beam. Further, the prior art calibration method taught by Heydemann does not compensate for the polarizing behavior of all of the optical elements, including in particular the beamsplitter that provides the signals for the two detectors. Finally, the Heydemann article does not teach a technique for calculating the relative strengths of the two orthogonal polarization components of the test beam.
Thus, although there is a significant advantage of the homodyne technique with respect to the light source, this advantage is often outweighed by the above mentioned concerns and difficulties. Consequently, it would be desirable to provide an improved homodyne receiver which could overcome or minimize these difficulties and concerns while still providing the advantages of the homodyne technique regarding the light source.
There is therefore an unmet need for an apparatus and method for high-speed, high precision measurement of the phase difference between two orthogonally polarized components of a test beam as well as the intensities of these two components. The present invention overcomes these disadvantages of the prior art by providing accurate values without the need for perfect optical elements and without requiring the type of specialized light sources needed for prior art heterodyne techniques.